1. Field of the Invention
The present invention relates to bevel gear mechanisms in a broad sense, including bevel gears in a strict sense, skew bevel gears, hypoid gears, etc., and to an electric power steering system using it.
2. Related Background Art
Such bevel gear mechanisms are used, for example, in the electric power steering systems and the like, and one of conventionally known configurations is a hypoid gear mechanism. The electric power steering system with the hypoid gear mechanism is provided with a hypoid pinion gear fixed to a rotational shaft of a driving motor and a hypoid ring gear fixed to a steering shaft, and the two gears are in mesh with each other in such a positional relation that their rotation axes are not parallel to each other and do not intersect with each other. This mechanism supplies a driving force of the driving motor as steering assistance to the steering shaft.
In such bevel gear mechanisms, it is also necessary to provide appropriate backlash for contact portions of the two gears in order to prevent seizure and breakage of tooth flanks. With too large backlash, it can result in noise such as gear jamming sound or the like. For example, Japanese Patent Application Laid-Open No. H07-232651 discloses a mechanism capable of adjusting the mount position of the hypoid ring gear relative to the steering shaft up and down along the axial direction, and this adjustment of the mount position leads to adjustment of magnitude of the backlash. Even with appropriate backlash, however, for example, where a moment of inertia is large on the hypoid pinion gear side including the driving motor, the noise such as gear jamming sound will occur readily at the contact portions of the bevel gear mechanism if the driving motor is actuated because of input from the tire side due to road undulations or the like (input of disturbance).
The present invention has been accomplished in order to solve this problem and an object of the invention is thus to provide a bevel gear mechanism that can more effectively reduce the noise such as gear jamming sound occurring at the contact portions of the bevel gear mechanism, and an electric power steering system using it.
In order to accomplish the above object, a bevel gear mechanism according to the present invention is constructed in structure comprising a first bevel gear of a ring shape having teeth projecting on one disk surface side; a second bevel gear of a ring shape arranged to be concentric with the first bevel gear and having teeth placed on a concentric circle basis on the same surface side as the teeth of the first bevel gear; a support member for substantially restraining rotation of the second bevel gear relative to the first bevel gear and for elastically supporting the second bevel gear relative to the first bevel gear so that in a free state the teeth of the second bevel gear can project relative to the teeth of the first bevel gear in a direction of a rotation axis and so that in a pushed state, when the teeth of the second bevel gear is pushed to the first bevel gear side in the direction of the rotation axis, the teeth of the second bevel gear can be retracted to the first bevel gear side; and a fluctuation suppressing member for suppressing fluctuation of the second bevel gear relative to the first bevel gear along a predetermined diameter direction.
Since the teeth of the second bevel gear elastically supported by the support member project relative to the teeth of the first bevel gear, when a third bevel gear described hereinafter is assembled together with the gears, the second bevel gear and the third bevel gear go in contact with each other in the assembled state and the third bevel gear pushes the second bevel gear to the first bevel gear side, whereby the second bevel gear is retracted to the first bevel gear side in the direction of the rotation axis thereof to deform the support member or the like. The second bevel gear under restoring force of the support member or the like gears with the third bevel gear while pushing it. If the third bevel gear is rotationally driven because of input from the tire side, kinetic energy of the third bevel gear at a start of rotation will be consumed by the degree of resisting the pushing force of the support member or the like, so as to reduce impact upon collision between teeth and thus suppress occurrence of gear jamming sound.
According to the results of intensive and extensive studies by the inventor, it became clear that, where the moment of inertia was large on the third bevel gear side and when the third bevel gear was rotationally driven because of the input from the tire side, the second bevel gear tended to fluctuate readily along a direction nearly perpendicular to an axis connecting the contact part of the third bevel gear and the center of rotation of the second bevel gear, with rotation of the third bevel gear. If the second bevel gear should fluctuate in this way, flexural rigidity of the support member would be insufficient and as a consequence, the consumption of energy of the third bevel gear would be insufficient at the start of rotation, which could cause the third bevel gear to collide with the first bevel gear to make noise. Therefore, the fluctuation suppressing member is provided to suppress the fluctuation of the second bevel gear along the predetermined diameter direction, which is the direction nearly perpendicular to the axis connecting the contact portion with the third bevel gear described hereinafter and the center of rotation of the second bevel gear.
The invention involves two positional relations between the first bevel gear and the second bevel gear; a relation wherein the first bevel gear is located on the inner periphery side and the second bevel gear on the outer periphery side, and a relation wherein the first bevel gear is located on the outer periphery side and the second bevel gear on the inner periphery side.
It is preferable that this fluctuation suppressing member be arranged so that a fixed end thereof is fixed to the first bevel gear while a free end thereof is in contact with at least one of the second bevel gear and the support member, thereby elastically supporting the second bevel gear and the support member relative to the first bevel gear.
This structure permits the fluctuation suppressing member to reinforce the elastic support force of the support member, which enhances torsional rigidity and flexural rigidity along the direction of fluctuation of the second bevel gear, so as to suppress the fluctuation of the second bevel gear relative to the first bevel gear. Since the elastic support force is thus enhanced, the energy upon collision can be absorbed by flexural of the support member and the fluctuation suppressing member even if the moment of inertia is large on the third bevel gear side, so that the impact is also relaxed upon collision between teeth.
In the bevel gear mechanism further comprising a third bevel gear capable of gearing with the first and second bevel gears, which has another rotation axis not parallel to that of the first and second bevel gears, the fluctuation suppressing member suppress the fluctuation of the second bevel gear along the direction nearly perpendicular to the axis connecting the center of rotation of the second bevel gear and the contact part between the second bevel gear and the third bevel gear.
In the gearing state with the bevel gear, the second bevel gear is easy to fluctuate along this direction. This tendency becomes great where the moment of inertia is large on the bevel gear side. Thus the fluctuation suppressing member mainly suppresses the fluctuation along this direction, whereby occurrence of noise is restrained while suppressing increase in rotational friction of the gear mechanism.
It is preferable herein that the fluctuation suppressing member be comprised of a pair of plate spring members holding the second bevel gear and the support member on the both sides thereof along the direction of the rotation axis of the first bevel gear.
When the fluctuation suppressing member is comprised of the pair of plate spring members to hold the second bevel gear and the support member, the torsional rigidity and flexural rigidity of the support member and the fluctuation suppressing member can be enhanced fully.
An electric power steering system according to the present invention is constructed in structure using the bevel gear mechanism of the above structure, in which the aforementioned first and second bevel gears are connected to a transmission shaft of steering force, for example, to a steering shaft, in which the third bevel gear is connected to an output shaft of an electric motor, and in which these bevel gears constitute a hypoid gear mechanism.
This electric power steering system is free of occurrence of vibration and the noise such as gear jamming sound due to the disturbance such as input from tires or the like, and provides steering feeling without anomaly.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.